1. Field of Invention
The invention relates to electronic flash devices having particular utility with low cost photographic cameras, and more specifically to charging and charge-control circuits for such flash devices.
2. Description of the Prior Art
Electronic flash devices typically include capacitors that are charged from a battery and discharged through a gas-filled flash tube. Energy from the discharging capacitor excites the gas, which illuminates the scene.
Design considerations usually involve a balance between a reasonably long battery life and the desire for rapid and continuous charging of the capacitor. In multiple use cameras this balance frequently is resolved in favor of continuous or automatically recycled charging whenever the flash is in a ready mode. If the battery is drained, there may be some inconvenience, but it is replaceable. In single use cameras, on the other hand, the batteries are seldom accessible for replacement. Elimination of undue battery drain is particularly important, even in the ready mode. Usually the operator is required to maintain continuous pressure on a biased switch. Charging stops when the switch is released, saving the battery.
One example of a recent approach for balancing the above-mentioned considerations is depicted in Konica Japanese Publication No. 3-65129U. A photographic camera is disclosed with an electronic flash device that has a charging cycle initiated by one switch and arrested by another switch. Momentary depression of the first switch energizes a self-oscillating charging circuit which continues charging after the momentary switch is released. An inductive coupling and capacitive timing circuit are used to activate the second switch and arrest the oscillations several seconds after recharging is completed. A ready lamp is coupled across the flash capacitor for visually indicating when the device has sufficient charge for satisfactory operation.
Although prior flash devices offer many advantages, the present invention addresses problems that remain, particularly in connection with low cost charging circuits and single use cameras. Continuous pressure on a biased switch may save the battery, but it also requires the operator's attention, which might better be directed to scene composition. Even in cameras having replaceable batteries, replacement is inconvenient and often is required in the middle of a transient photographic event.
The solution proposed in the above-mentioned publication offers unattended charging and automatic shut-off, but relies on indirect inductive coupling and a capacitive timing circuit. Tolerance variability in such components and circuits is not conducive to reasonably precise yet inexpensive charge control. Temperature changes effect circuit characteristics and degrade performance. Closely controlled operation with a ready light also is difficult, since the ready light works directly off the capacitor, while the shut-off control is inductively coupled.